A user apparatus or user equipment (UE) in a mobile communication system changes its operating mode from an active state to an idle state after a call is finished. The active state may be referred to as a connection state, and the idle state may be referred to as a standby state. In the standby state, user apparatuses are not managed as to which cells they are residing in but managed as to which tracking areas (TAs) they belong to. The tracking area (TA) may be referred to as a location registration area, a routing area (RA), a location area (LA) or others and is an area where paging is concurrently performed upon incoming calls. The TA may be managed by an upper node such as a MME/UPE to a base station eNB.
FIG. 1 schematically illustrates a mobile communication system. In the illustration, the first tracking area TA1 includes three cells (cell 1, cell 2, cell 3), and the second tracking area TA2 includes two cells (cell 4, cell 5). It is assumed that a user apparatus UE residing in cell 1 has finished a call and entered the standby state. In general, base stations for cells broadcast cell identification information (cell ID) and tracking area identification information (TA-ID) as broadcast information (BCH). For this reason, the user apparatus UE would know that the TA-ID for the camped cell is for TA1. It is assumed that the user apparatus has moved to cell 3 while being kept in the standby state. Then, the user apparatus UE receives the broadcast information from a base station for cell 3 and can recognize the cell identification information and the TA-ID. Since cell 1 and cell 3 belong to the same tracking area (TA1), no tracking area change request is issued. Furthermore, it is assumed that the user apparatus UE has moved from cell 3 to cell 5 while being kept in the standby state. Then, the user apparatus UE receives the broadcast information (BCH) from abase station for cell 5 and can recognize that the tracking area has changed from TA1 to TA2. Upon recognizing this change, the user apparatus UE transmits a request for changing the tracking area to the base station for cell 5. Then, the base station transmits a signal for requesting update of the tracking area to an upper node such as a MME/UPE and updates the tracking area for the user apparatus UE from TA1 to TA2. Subsequently, when calls for the user apparatus UE are incoming, calling (paging) is performed in the tracking are TA2. In this manner, the update of tracking areas (tracking area update (TAU)) is performed in response to requests from user apparatuses. Note that one or more tracking areas may be associated with one user apparatus as in the case of next generation mobile communication systems such as LTE (long Term Evolution) systems.
In recent years, cells called closed subscriber group (CSG) cells as illustrated in FIG. 2 have been increasingly used. A network including the CSG cells may be referred to as a private network (P-NW), a home network or others. The CSG cell may a mobile communication network which only company staff members in a certain company can access within a building of the company, a mobile communication network which only family members can access, a mobile communication network accessible within a school ground, a mobile communication network accessible within a certain shop or other mobile communication networks. For example, user apparatuses possessed by company staff members can communicate in the CSG cell within the company and communicate in a macrocell outside of the company building. A network using such a macrocell may be also called a public network and be a public mobile communication network rather than a private network. Users that are not staff members of that company are prevented from communicating in the CSG cell within the building.
As a result, a cell reselected by a user apparatus operating in the idle state may be a macrocell or a CSG cell. It is preferred that a user belonging to any CSG be in standby in the associated CSG cell. In other words, it is preferred that a moving user apparatus reselect the CSG cell which the user apparatus belongs to. For example, in a document R2-072825 “Triggering of measurements in LTE_IDLE for CSG Cells” Vodafone, some techniques are disclosed for efficiently searching for CSG cells through distinction between locations where the CSG cells are likely to be present and locations where the CSG cells are not likely to be present.